1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus using the same. More specifically, the invention relates to reduction in pitch unevenness in an optical scanning device where a flux of light emitted from light source means enters a deflection surface of deflecting means at a given angle in the sub-scanning section and the flux of light reflected and deflected by the deflecting means is applied to a recording medium (photosensitive drum) surface to record image information. The optical scanning device of the present invention is suitable for an image forming apparatus that involves an electrophotographic process, such as a laser beam printer, a digital copying machine, and a multi-function printer.
2. Related Background Art
Various multi-beam optical scanning devices which achieve multi-color by making fluxes of light enter into a common scanning optical system have been proposed in the past. To irradiate surfaces to be scanned with fluxes of light separately in this type of multi-beam optical scanning device, the light fluxes have to be separated after deflected and reflected by a light deflector (e.g., polygon mirror), and if sources of the light fluxes have the same wavelength, spatial separation is necessary. Objective spatial separation is achieved by, for example, making the fluxes of light enter into a deflection surface (reflection surface) of a light deflector from an oblique direction in the sub-scanning section (hereinafter referred to as oblique incidence).
FIGS. 15A and 15B are partial sectional views showing oblique incidence in an optical system of a multi-beam optical scanning device. FIG. 15A shows the main scanning section whereas FIG. 15B shows the sub-scanning section perpendicular to the paper plane of FIG. 15A. Illustrated in FIGS. 15A and 15B are two fluxes of light entered into a deflection surface of a light deflector from opposing directions and the behavior of the light fluxes after they are deflected and reflected by the deflection surface.
In FIGS. 15A and 15B, reference numeral 91 denotes a light deflector, for example, polygon mirror. 92a and 92b denote two oblique incident rays deflected and reflected by the polygon mirror 91. 95 denotes a scanning lens system (fθ lens system) composed of a cylindrical lens 93 and toric lenses 94a and 94b (a two-lens system). The cylindrical lens 93 has a given refractive power only in the main scanning section.
An incidence optical system, which is not shown in FIGS. 15A and 15B, makes two fluxes of light enter obliquely into the polygon mirror 91. In the incidence optical system, two light fluxes are emitted from two light sources (the number of light sources corresponds to the two oblique incident rays 92a and 92b), are converted into parallel pencils of light by a collimator lens, and then form a linear image in the vicinity of a deflection surface 91a of the polygon mirror 91 with the help of a cylindrical lens that has a given refractive power only in the sub-scanning section. This is a measure taken to correct a tilt of the polygon mirror's deflection surface within the sub-scanning section by placing the polygon mirror's deflection surface and a surface to be scanned (photosensitive drum surface) in an optically conjugate relation. In short, the incidence optical system constitutes a deflection surface tilt correction optical system.
The two light fluxes deflected and reflected by the polygon mirror 91 (the oblique incident rays 92a and 92b) are led to given positions on surfaces of photosensitive drums by the scanning lens system 95 through their respective light path-bending mirrors (not shown in the drawings). As the polygon mirror 91 is rotated, the rays draw scanning lines in the axial direction (main scanning direction in which deflection scan with the light fluxes proceeds) and rotation of the photosensitive drums which is synchronized with the rotation of the polygon mirror 91 arranges the scanning lines at regular intervals in the sub-scanning direction perpendicular to the main scanning direction.
In this way, surfaces of two photosensitive drums can simultaneously be scanned. If a scanning optical system is placed on each side of the polygon mirror to share the polygon mirror between the two scanning optical systems, exposure and development can be finished for four colors at once, thus making high-speed, full-color printing possible.
However, the principle of separation of rays through oblique incidence makes variation in distance from a rotation axis to deflection surfaces inevitable due to polygon mirror's manufacture error and the like (the difference is hereinafter referred to as “deflection surface eccentricity”). The deflection surface eccentricity makes the pitch irregular and leads to poor image reproducibility.
Therefore, while a normal scanning optical system only has to deal with pitch unevenness caused by a tilt of a deflection surface of a polygon mirror, an oblique incidence optical system has to solve pitch unevenness caused by deflection surface eccentricity in addition to pitch unevenness caused by a tilt of a deflection surface of a polygon mirror.
Although a multi-beam optical scanning device is taken as an example in the above description of prior art, conventional single-beam optical scanning devices that use oblique incidence also have the same problem.
Prior art has proposed a diversity of optical scanning devices in which pitch unevenness due to deflection surface eccentricity is reduced (see Japanese Patent Application Laid-Open No. H10-327302 and Japanese Patent Application Laid-Open No. 2001-051226, for example).
Japanese Patent Application Laid-Open No. H10-327302 discloses a technique that reduces, in the case of on-axis incidence, pitch unevenness resulting from polygon mirror's deflection surface eccentricity by setting parameters in a manner that limits the pitch unevenness to within ¼ pixel. Japanese Patent Application Laid-Open No. 2001-051226 discloses a technique that reduces, in a system where a flux of light has a given incident angle also in the main scanning section, pitch unevenness resulting from polygon mirror's deflection surface eccentricity by setting parameters in a manner that limits the pitch unevenness to within ⅕ pixel to 1/10 pixel.
Japanese Patent Application Laid-Open No. H10-327302 and No. 2001-051226 both state that pitch unevenness caused by a tilt of the deflection surface can be ignored because of a deflection surface tilt correction system provided. In actuality, however, no deflection surface tilt correction system has ever truly succeeded in correcting the tilt throughout the entire scan region since the deflection surface wobbles as the polygon mirror rotates. Furthermore, the conjugate point could be shifted by error in manufacture, arrangement and the like of an imaging lens and other components, thereby deteriorating further deflection surface tilt and increasing the pitch unevenness.